1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and a packaged body of the lithographic printing plate precursors. More specifically, the invention relates to a lithographic printing plate precursor capable of direct plate making by scanning with laser beams on the basis of digital signals such as computer data, and a packaged body of the lithographic printing plate precursors.
2. Description of the Related Art
A lithographic printing plate generally comprises a lipophilic image area that receives ink and a hydrophilic non-image area that receives a fountain solution in a printing process. Lithographic printing is a printing method of making difference in ink-adhering property on the surface of a lithographic printing plate with the lipophilic image area of the lithographic printing plate as the ink-receptive area and the hydrophilic non-image area as the fountain solution receptive area (ink-repellent area) by making use of the natures of water and oily ink of repelling to each other, adhering ink only on the image area, and transferring the ink to the material to be printed, e.g., paper.
For manufacturing the lithographic printing plate, a lithographic printing plate precursor (a PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (an image-recording layer) has so far been widely used. The lithographic printing plate is generally obtained by a plate-making method of exposing a lithographic printing plate precursor through an original image of a lith film and the like, and then, for leaving the area to become an image area of the image-recording layer, dissolving and removing other unnecessary image-recording layer with an alkali developing solution or an organic solvent, to thereby bare a hydrophilic support surface to form a non-image area.
In a conventional plate-making process of a lithographic printing plate precursor, a process of dissolving and removing unnecessary image-recording layer with a developing solution and the like after exposure is necessary, but the exclusion or simplification of such an additional wet process is one of the objects in the industry. Since the discard of waste solutions discharged with wet processes is a particularly great interest in the industry at large in recent years from the consideration of the global environment, the solution of the above problem is increasingly desired.
As a simple plate-making method, a method that is called on-press development is proposed, which is a method of using an image-recording layer capable of removing an unnecessary area of a lithographic printing plate precursor in an ordinary printing process, and removing a non-image area after exposure on a printing press to obtain a lithographic printing plate.
As the specific examples of on-press development, e.g., a method of using a lithographic printing plate precursor having an image-recording layer soluble or dispersible with, e.g., a fountain solution, an ink solvent, or an emulsified product of a fountain solution and ink, a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket of a press, and a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket after weakening the cohesive strength of an image-recording layer or the adhesive strength of an image-recording layer and a support by the permeation of a fountain solution and an ink solvent are exemplified.
In the present invention, unless otherwise indicated, “development process” means a process of removing an unexposed area of an image-recording layer of a lithographic printing plate precursor by being brought into contact with a liquid (generally an alkali developing solution) to thereby bare the hydrophilic support surface with an apparatus other than a printing press (generally an automatic processor), and “on-press development” means a method and a process of removing an unexposed area of an image-recording layer of a lithographic printing plate precursor by being brought into contact with a liquid (generally a printing ink and/or a fountain solution) to thereby bare the hydrophilic support surface with a printing press.
On the other hand, in recent years, digitized techniques of electronically processing image data, accumulating and outputting with a computer have prevailed, and various image output systems corresponding to these digitized techniques have been put to practical use. Under such circumstances, a computer-to-plate technique directly making a printing plate is attracting public attention, which technique comprises scanning exposing a lithographic printing plate precursor with high convergent radiant rays such as laser beams carrying digitized image data without using a lith film. With such a tendency, it is an important technical subject to obtain a lithographic printing plate precursor well adapted to such a purpose.
Accordingly, in recent years, the simplification of plate-making operation, and the realization of dry system and non-processing system have been further strongly required from both aspects of the above-described global environmental protection and the adaptation for digitization.
To cope with this problem, there is disclosed in Japanese Patent No. 2938397 a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer containing hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder. Japanese Patent No. 2938397 discloses that it is possible to perform on-press development with a fountain solution and/or ink by subjecting the lithographic printing plate precursor to exposure with infrared laser beams to coalesce the hydrophobic thermoplastic polymer particles by heat to thereby form an image, and then mounting the lithographic printing plate precursor on the cylinder of a printing press.
However, although a method of forming an image by coalescence of fine particles by mere heat fusion as above certainly shows good on-press developability, there are problems that image strength is weak and press life is insufficient.
Therefore, it is proposed to improve press life by making use of polymerization reaction. For example, a lithographic printing plate precursor having an image-recording layer (a heat-sensitive layer) containing microcapsules encapsulating a polymerizable compound on a hydrophilic support is disclosed in JP-A-2001-277740 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”.) JP-A-2002-287334 discloses a lithographic printing plate precursor comprising a support having provided thereon an image-recording layer (a heat-sensitive layer) containing an infrared absorber, a radical polymerization initiator and a polymerizable compound. JP-A-2000-39711 discloses a lithographic printing plate precursor capable of on-press development after exposure, which lithographic printing plate precursor comprises an aluminum support using thereon a photosensitive composition containing (a) a water-soluble or water-dispersible polymer, (b) a monomer or an oligomer having a photopolymerizable ethylenic unsaturated double bond, and (c) a photo-polymerization initiator having an absorption maximum in the ultraviolet region. These methods using reactions such as polymerization reaction are characterized in that the density of chemical bonding in an image area is high, so that image strength can be increased.
In general, as the preprocess of mounting a printing plate on a press on printing site, the inspection and the discrimination of images on a printing plate, i.e., works for ascertaining whether the images fitting for the purpose are recorded on the printing plate or not, and ascertaining for what a color of ink the plate is, are operated. In ordinary lithographic printing plate precursors accompanied by a development process, an image can be easily ascertained after plate making (after development process), or before printing (before a printing plate is mounted on the press) generally by coloring an image-recording layer in advance.
However, since there is no image on a printing plate in a lithographic printing plate precursor of an on-press development type or a non-processing (non-development) type not accompanied by development process before printing, the discrimination of a plate cannot be done, which sometimes leads to the error in operation. In particular in multicolor printing, it is important for printing work to be capable of distinguishing whether register marks for register are clearly written so as to be distinguished or not.
To cope with this problem, there is disclosed in JP-A-11-277927 a photosensitive lithographic printing plate containing a light-heat converting agent having absorption in the wavelength region of 600 nm or more, a compound capable of generating at least one selected from an acid, a base and a radical by light or heat, and a compound that discolors by the interaction with at least one of the generated acid, base and radical. However, although the inspection work becomes easy by using in an image-recording layer the combination of a leuco dyestuff with a color developer or the combination of a light-acid generator with a pH indicator as the specific examples of the discoloring compound disclosed in JP-A-11-277927, drawbacks arise such that paper loss increases and printing performances are adversely influenced such that soiling resistance and press life are deteriorated, since a compound that does not concern image formation must be added for generating discoloration.
Further, in a lithographic printing plate precursor having a radical polymerizable image-recording layer that is subjected to development processing, a protective layer is generally provided on the image-recording layer for the purpose of polymerization acceleration (for achieving higher sensitivity and higher press life) by oxygen shielding, and prevention of the occurrence of scratches and ablation due to high intensity laser exposure. A water-soluble polymeric compound excellent in crystallizability, e.g., polyvinyl alcohol, is conventionally used as the main component in such a protective layer. However, for sufficiently exhibiting the function, a relatively great amount of a protective layer must be provided on the image-recording layer. Even if a great amount of a protective layer is provided on the image-recording layer, the protective layer is all removed at the time of development with a lithographic printing plate precursor accompanied by development, so that there arises no problem in printing. However, with an on-press development type lithographic printing plate precursor, such a great amount of a protective layer causes the problems of delay of on-press development, mixing of removed substances in a fountain solution, and accumulation on an impression drum, so that a protective layer capable of exhibiting the same function as conventional protective layers is required.
A lithographic printing plate precursor having a image-recording layer containing a radical polymerization initiator, a polymerizable compound and a polymer binder and further having provided thereon a protective layer is disclosed in U.S. Patent Application 2003/0,064,318A1. There is disclosed in the same patent that a polymer having a polyethylene oxide segment is used as the polymer binder, and the lithographic printing plate precursor is applicable to on-press development. However since the prescription of the protective layer in U.S. Patent Application 2003/0,064,318A1 is the same as those of conventionally used protective layers, when the coating solution of the protective layer is coated on the image-recording layer, the image-recording layer partially dissolves by the protective layer coating solution and the image-recording layer and the protective layer are mixed. Therefore, there is a problem in U.S. Patent Application 2003/0,064,318A1 that the image-forming property is rather hindered by coating the protective layer and sensitivity is reduced.
For preventing mechanical shock during preservation of lithographic printing plate precursors, lithographic printing plate precursors are generally preserved and transported in the state that an interleaf is inserted between printing plate precursors. Interleaving sheets are generally manufactured from wood pulp, synthetic pulp (polyethylene fiber), natural fibers, e.g., hemp, reclaimed cellulose and the like. Interleaving sheets are necessary during preservation and transportation but they are not necessary at all after lithographic printing plate precursors are exposure processed, and generally they are recycled or discarded. With the increase of the consciousness of environmental protection in recent years, lithographic printing plate precursors that do not necessitate interleaving sheets are required in the printing industry.
Lithographic printing plate precursors that do not necessitate development process and free from interleaving sheets are required as described above. However, if polymerization reaction is applied to lithographic printing plate precursors that do not necessitate development process, the lithographic printing plate precursors are adhered to each other when they are laminated without using interleaving sheets, since the content of soft polymerizable compound accounting for in a photosensitive composition is high, and the image-recording layer or the protective layer is scratched, and aging stability and press life are deteriorated.